The present invention relates generally to the field of security systems, and more particularly, to an improved passive infrared radiation detection device of the type commonly used in conjunction with such security systems.
It is presently common practice for state of the art security systems to incorporate not only a so-called "outer perimeter" comprised of appropriate switching devices for detecting access through openings (e.g., door, window, etc.) of a protected premises, but also one or more devices stationed within the premises to detect intruders which may have been successful in penetrating the outer perimeter of the security system. These are often referred to as "motion detectors", and include any of a variety of available devices for detecting motion within a defined region.
One such device which has found broad acceptance and applicability to the detection of motion within a defined region of a premises is the "passive infrared radiation" detector (hereafter referred to as a "PIR" detector) which, as its name suggests, is capable of detecting levels of infrared radiation. These detected levels of infrared radiation can then be interpreted to identify movements within the defined region. A PIR detection system of this general type, consisting of a PIR detector and preamplifier circuit, may be obtained commercially from suppliers such as Ademco and Racal, among others.
Considering their intended use (i.e., in a security system), much effort has been expended in improving the reliability of such state-of-the-art PIR detection systems by improving their immunity to noise and their ability to distinguish between valid and false alarms. Specifically, extensive efforts have been made to remove contributing, internally generated disturbances such as "popcorn" noise, or external radio frequency interference, and other types of electronic noise, through appropriate improvements in circuit design. While these improvements have significantly increased the reliability of such devices, the potential for further noise suppression (and improved "catch" sensitivity in detecting an intruder) is rather limited, and only marginally cost effective. Consequently, the overall quality of available PIR detection systems (dictated to a large extent by the measures available to improve noise rejection and increase sensitivity) has reached somewhat of a plateau in terms of performance.
Another problem which has been encountered in using state-of-the-art PIR detection systems is their limited ability to distinguish between a valid object to be detected (i.e., an intruder) and other "legitimate" images which may come to be detected such as pets or even rodents, light and heat sources present in the monitored region, or even the flashing headlights of automobiles, among others. Interesting to note is that even if it would be possible to further improve the noise rejection and "catch" sensitivity of the PIR detection system, false alarms could still not be entirely eliminated due to the inability of the PIR detection system to distinguish between an intruder and other, legitimate images present in the region that is being monitored.
It therefore became desirable to develop a new approach toward the improvement of a passive infrared radiation detection device which would not only serve to further suppress noise and improve sensitivity, but which would also assist in better identifying intruders to reduce false alarms.